An electroacoustic balun device is known from U.S. Pat. No. 7,091,649. The balun comprises two stacks, each with two resonant piezoelectric layers, one above the other, with electrodes in the stack on either side of each layer. In operation, voltages are applied between the electrodes of the resonator in a first one of the piezoelectric layers. This results in acoustic vibrations in the resonator in the first piezoelectric layer, which are coupled to the second one of the piezoelectric layers so that vibrations also occur in the second piezoelectric layer. Due to the piezoelectric effect this in turn results in a voltage across the electrodes of the second piezoelectric layer.
U.S. Pat. No. 7,091,649 describes a device with two of such stacks, using separate electrodes. In operation, voltages are applied between the electrodes of the resonators of both stacks in the first piezoelectric layer, to produce voltages between the electrodes of the resonators in the second piezoelectric layer. The polarization of the piezoelectric layers is set differently in the different stacks, in order to create a phase relation between the resulting voltages from the resonators in the second layer so that the series connection of these resonators produces twice the voltage applied to the series connection of the resonators in the first layer. Thus balun functionality can be realized.
Polarization differences in a piezoelectric layer have also been used for other purposes. U.S. Pat. No. 3,018,415 discloses a resonator comprising a piezoelectric crystal with two electrodes on respective sides of the crystal. In order to suppress undesirable vibration modes, at least one of the electrodes is shaped into a central disc and a surrounding ring, and the polarization of the crystal is set in mutually opposite directions below the disc and the ring respectively.
U.S. Pat. No. 5,576,590 discloses an electroacoustic transformer with a driver part and a generator part. The transformer comprises a piezoelectric plate. In the driver part, electrodes are attached on mutually opposite sides of the plate to excite acoustic vibrations. These vibrations travel outward from the driver part as an acoustic wave with longitudinal polarization (i.e., with vibrating movement in the direction of propagation in the plane of the plate). The generator part comprises electrodes at different positions on the plate. In the generator part, the plate is polarized between these positions in order to convert the waves into electric fields in the plane of the plate between the electrodes.
U.S. Pat. No. 6,448,695 discloses a frequency bandpass filter that is formed using a series arrangement of acoustic resonators. Coupling between the resonators is realized acoustically rather than electronically, by means of lateral wave coupling.